Offshore platforms are used to provide stable and safe locations above the ocean surface for drilling and other operations associated with the exploration and extraction of oil and gas resources. While offshore platforms have been used by the oil and gas industry for many years in relatively shallow waters, such as the Gulf of Mexico or the North Sea, the increasing demand for energy has created the need to exploit oil and gas resources from deepwater locations. Many of the traditional offshore platform designs used for shallow water applications are not practically adaptable for use in deeper waters. In addition, the platform designs that are in use, or which have been proposed for use in deep water locations have various disadvantages and limitations.
The design of offshore platforms presents many structural engineering challenges. Such platforms are subjected to severe environmental forces associated with the movement of the surrounding water and air. The platform responds to these forces by moving, to some degree, in several ways, including horizontal movement along the surface in direct response to an applied force, rolling (side-to-side rocking along an axis in the direction of the prevailing current), pitching (side-to-side rocking along an axis perpendicular to the direction of the prevailing current), yawing (rotation about the vertical), heaving (up and down motion), surging (an offset in the direction of the current about the anchorage), and swaying (an offset sideways about the anchorage). The structure must be able to withstand periodic forces that are capable of inducing vibration, possibly causing at oscillating frequencies of the structure. These movements, while unavoidable, must be constrained within acceptable limits by the structural design of the platform. This, in turn, imposes limitations on the various components used in the design. The limits on what constitutes acceptable movement of the platform is normally determined by the nature of the operations that are intended to occur on or near the structure, such as the operation of drilling equipment and the docking of ships or landing of helicopters on a platform, the protection of risers from the seabed to the platform, and the support of risers that pass into the seabed. The structure and any occupants must also be able to safely ride the high winds and seas of storms.
Deepwater platforms in use, or which have been proposed, include (1) tension leg platforms (TLPs) that are fixed at a location with generally vertical tendons anchored to the seabed that are in tension and are connected to a floating platform, (2) catenary moored systems such as semi-submersible floating structures and spar-like floating structures that are stabilized with cables anchored to the seabed and forming a catenary between the floating platform, and (3) buoyant leg structure (BLS), sometimes referred to as a buoyant “pile” structure. Buoyant leg structures are described in the following U.S. patents, incorporated herein by reference: U.S. Pat. Nos. 5,118,221, 5,443,330, and 5,683,206 to Copple, and U.S. Pat. No. 6,012,873 to Copple et al. (the “Copple patents”). For reasons described in the Copple patents, the buoyant upper portions of a BLS provide added stability against environmental forces.
There are several features that are common to buoyant leg structures. A BLS includes one or more hollow members that form a column that extends downwards from the surface of the water towards the seabed. The hollow members can be formed, for example, from stacked compartments or from an elongated hollow member, such as a pipe or tube. The column is anchored to the seabed, either directly or by a tether. The hollow members have a lower portion that is partially filled with seawater or can be used for storing oil, and an upper portion that is emptied to provide predetermined buoyancy. For BLSs formed from elongated hollow members, a watertight bulkhead provides partitioning between the lower portion and upper portion. The center of buoyancy is above the center of gravity, so that when the top of the BLS is displaced by currents or winds, a righting moment tends to straighten the BLS. Another characteristic of a BLS is that the lower portion of the BLS is in tension and the upper portion of the BLS is in compression.
While the BLS designs described in the Copple patents disclose structures wherein the buoyant leg is directly anchored to the seabed, subsequent BLS designs contemplated by the inventors are anchored by a tether enabling use at water depths much greater than alternative deepwater platforms-perhaps to depths of 10,000 feet or more. At these greater depths, the natural oscillating period of a BLS increases in heave, and may correspond to periods having substantial wave energy. When this occurs, energy from the waves can couple into the BLS, producing large up and down platform motions. Prior buoyant leg structures have a limited ability to design around this problem.
Therefore, it is one aspect of the present invention is to provide a BLS having added stability in deep water.
It is another aspect of the present invention is to provide an offshore deep-water platform suitable for use at great depths that has increased buoyancy.
It is yet another aspect of the present invention is to provide an offshore deep-water platform suitable for use at great depths that has increased buoyancy for supporting heavier platforms.
It is one aspect of the present invention to provide an offshore deep-water platform that is less susceptible to vortex shedding and to vortex induced vibrations.
Another aspect of the present invention is to provide an offshore deep-water platform that is simple in design, and which is relatively easy and inexpensive to construct, moor and operate.